Beat the butter and peanut butter: In a large bowl, beat softened butter and creamy peanut butter on medium speed for 2–3 minutes until smooth and creamy.

Add the powdered sugar: Gradually add powdered sugar, about 1/2 cup at a time, beating well after each addition until the mixture thickens.

Mix in the vanilla: Add the vanilla extract and beat until fully combined. The frosting should look fluffy and smell rich and nutty.

Adjust the consistency: If too thick, add milk one tablespoon at a time, beating after each addition, until the frosting is creamy and spreadable.

Frost and enjoy: Use immediately to frost cakes, cupcakes, or brownies, spreading generously for a smooth, dreamy finish.